1. Field of the Invention
The present invention relates to a process for the production of polyester articles in a continuous melt-to-mold process, including a recycling step. A polyester melt composition is prepared from polyester precursors and directly molded or otherwise formed into useful shaped articles in a single, integrated process without solidifying the melt prior to molding. Recycled scrap polyester from the molding step is added to the polymer melt stream at any of a variety of points in the manufacturing process.
2. Description of the Prior Art
Polyesters are useful in a wide variety of applications, many of which require high molecular weight polyester to achieve acceptable properties. In this regard, it is known in the art that polyesters may be used for the manufacture of molded articles, such as food and beverage containers. The conventional method for preparing high molecular weight polyester articles involves melt-phase production of a precursor polyester to a moderate molecular weight followed by pelletization of the amorphous polymer; crystallization and solid-state polycondensation to increase molecular weight to the desired level; and remelting and molding of the polyester to form the desired article.
Polyester molding processes generate much scrap, which is generally reused to reduce costs. Conventionally, the scrap can be recycled to the molding process, which requires drying and leads to increased polymer degradation by increasing the time that the high molecular weight polyester is molten. Alternately, the scrap can be recycled by being transported and fed to a polyester manufacturing plant that produces pellets, fiber, or film. An integrated process for continuously producing formed articles starting from polyester precursor starting materials, melt, molding and recycling scrap polyester such that little or no polyester waste is generated is heretofore unknown.
The prior art describes a continuous melt phase process for the production of shaped articles such as bottle preforms, but it does not address recycling waste polyester generated in the molding step back to the polyester polymer formation stage. Methods for feeding polyester scrap to melt-phase polyester reactors during resin production are known. However, these are not concerned with integrated processes for producing molded articles. For example, U.S. Pat. No. 3,703,488 describes a process in which scrap is mixed with polyester monomer in an extruder, and then breaking it down to a degree of polymerization of 20 to 30 which can be pumped to a reactor. In U.S. Pat. No. 4,138,374, scrap is fed just prior to the outlet of a final melt polymerization reactor. German patents DE19503053 and DE 19505680 describe continuous melt-to-mold processes, but also fail to address the incorporation of recycled scrap polyester to the polymer formation step.
It has been unexpectedly found that it is now possible to provide a process whereby shaped articles may be prepared in a continuous process starting from polyester precursors, wherein recycled scrap polyester from the molding step is added during the polyester formation or molding steps. As a result, a process is attained which achieves high yields, approaching 100%, with its associated reduced cost of production and less waste for disposal. No drying of preforms is necessary prior to recycling the scrap, which is ordinarily required in order to prevent hydrolytic degradation. Any hydrolytic degradation that occurs in feeding the polymer to the polymerization process has little effect since the polymer undergoes further polycondensation. The scrap need not spend significantly more time in the molten state at high molecular weight. This reduces polymer degradation, which improves color and reduces acetaldehyde formation. It is also not necessary to transport the scrap to a separate polyester manufacturing processes.